1. Field of the Invention
The present invention relates to a flow control method and apparatus. More specifically, the present invention relates to a flow control method and apparatus for a data transmission between terminals during a congestion state in an ATM network having a network arrangement in which terminals connected to an existing LAN via the ATM network are interconnected.
2. Description of the Related Art
Heretofore, a flow control method and apparatus in an ATM network has been used, for example, as disclosed in Japanese Patent Laid-open No. 7-66820/1995. As disclosed in Japanese Patent Laid-open No. 7-66820/1995, a control is performed so as to stop a cell inflow from a user who transmits a cell causing a state of congestion, the cell inflow being stopped during the congestion state in a communication network. By this conventional method, a discarding of a cell (or cells) during the congestion state is avoided, and a retransmission of the discarded cell (or cells) is avoided.
FIG. 6 is a block diagram showing an example of a conventional flow control system. Numerals 61 through 65 denote user side units that are control sections possessed by the users. Numeral 66 denotes a network side unit and relates to a flow control on a switch side. Numerals 67 through 70 denote traffic control units in the network side unit 66, which are present for each input/output port of an N.times.N switch 71. Numerals 72 and 73 denote output buffers which are present for each input/output port of the switch 71. Numeral 74 denotes a cell discardability determining section for detecting a state of congestion of an output buffer and for informing the traffic control units 67 through 70 of the congestion detection. The state of congestion of the output buffer may be made by determining that the buffer has less than a certain amount of unused space currently available (i.e., more than 90% full). Numerals 75 through 78 denote transmission paths for connecting the network side unit to the user side unit. In general, there are a plurality of traffic control units, and the user side units may be connected in a ring configuration, as are units 61 and 62.
Next, an operation of a conventional flow control system and method will be described. When the user side units 61 through 65 detect a particular cell having reset instruction information (called a "reset cell") that is sent to the user side units by the traffic control units 67 through 70, an authorization to transmit a predetermined number of cells (e.g., window size) is given to the user side units. The predetermined number of cells are then transmitted to the transmission paths 75 through 78 by the user side units prior to the detection of another reset cell. When the traffic control units 67 through 70 confirm that all the user side units 61 through 65 cannot transmit any more cells or that no data to be transmitted exists, a reset cell is transmitted to the user side units. Also, when a reset cell timer expires, a reset cell is transmitted to the user side units to maintain a particular cell transfer rate. Not shown in FIG. 6 are sources providing data to the user side units via local area networks or the like. The user side units 61-65 convert the data into data packets that are sent out on the transmission paths 75-78.
When one or more of the cell buffers 72 and 73 are congested, the cell discardability determining section 74 signals the traffic control units that provide cells to the congested cell buffers, so as to inform them of the need to limit a cell inflow. Based on this, the traffic control units stop or slow down the rate of the transmission of the reset cell to the user side units, so as to control (that is, slow down or stop) the cell transmission rate by the user side units.
A first problem with the conventional flow control system as described above is that a throughput between user terminals is reduced in a network arrangement that provides connectivity between LANs using an existing network such as an Ethernet, with inter-LAN connectivity being via an ATM network.
The reason for this problem is that the control of the flow control system is performed within the ATM network. During a state of congestion, a data output is controlled, not in the terminal of the existing LAN, but in a user side unit (gateway) terminating the ATM network. Thus, data continues to be transmitted from the terminal even during the state of congestion. When the control is performed so as to stop the cell transmission from the gateway to the ATM network, the data from a transmitting terminal is accumulated in a buffer in the user side unit. The transmitted data is transmitted after a considerable delay. Furthermore, the transmitted data may be discarded due to an overflow of the buffer.
A second problem with the conventional flow control system is that a time lag occurs during a time interval from a detection of congestion to a stop of data transmission. As a result, when a plurality of user side units are present, even if the state of congestion is detected, the control is not immediately started. It may thus take some amount of time to release the congestion.
The second problem occurs since an active control for a user side unit is not performed during the state of congestion. During the state of congestion, the traffic control unit only stops an information transmission that instructs the user side unit to reset a transmission disabled state. Therefore, until each user side unit finishes outputting the transmittable number of cells, If the flow control is not started.